1. Field of the Invention
This invention relates generally to injection molding, and in particular, to the use of a single hot-runner valve gate to mold upper and lower shells for mini-compact disk (mini-disk) cartridges, and the like.
A mini-disk cartridge is used to store information and is comprised of several basic components. A media storage disk, commonly referred to as a mini-compact disk, is stored within a shell which protects the disk from harmful debris. The shell is typically comprised of a lower shell and an upper shell. When manufacturing the mini-disk cartridge, the disk is placed into the lower shell and the upper shell is then placed over the lower shell and disk. The lower and upper shells are fastened together using internal bosses. Read/write ports are incorporated into the lower and/or upper shells to allow a reading/writing device to access the mini-compact disk within the shell. A sliding shutter covers the read/write port when the mini-disk cartridge is not in use to protect the mini-compact disk from harmful debris.
2. Description of the Prior Art
As is illustrated in FIG. 1 of the prior art, two pairs of juxtaposed pinpoint gates 10, 20 and 30, 40 are needed to injection mold both the lower shell 50 and upper shell 60, respectively, of a mini-disk cartridge. This pinpoint gate arrangement assures that cavities within a mold associated with shutter recess 70, read/write port 90, shutter guide track 80, fastening bosses on inner surfaces (not shown), and other tight corners and small crevices are filled quickly and constantly, to insure that the cooling time for the plastic is the same throughout the mold. As long as the cooling time throughout the mold is the same, the possibility of warpage within the shell is significantly reduced.
There are several disadvantages associated with using two cold-runner pinpoint gates to injection mold shells of a mini-disk cartridge, or the like. One such disadvantage is the accumulation of polymer string at pinpoints 11, 21, 31, and 41. This string must be removed from the shell or further manufacturing problems may occur when other components of the mini-disk are placed within the shell. Furthermore, at least two pinpoint gates are necessary to mold each shell because the flow rate through a single a pinpoint gate is inadequate to fill up the cavity fast enough for a uniform cooling time. If only one cold-runner pinpoint gate is used, mold filling time increases, a polymer "skin" is formed from the melt which clogs the mold, shell walls of non-uniform thickness are formed, and small cavities in the mold associated with shutter recess 70, read/write port 90, shutter guide track 80, the fastening bosses, and other small cavities are prevented from filling, thus producing a defective product. Additionally, excessive warpage of the shells occurs due to uneven cooling of the plastic when only one cold-runner value gate is used.
Alternatively, two hot-runner valve gates have been utilized to decrease mold filling time for uniform cooling. However, it is difficult to balance the cooling temperature of the two valve gates, necessitating constant adjustments and tweaking during the manufacturing process, thus decreasing the overall time efficiency of the molding process.